Written by Brad Garner and narrated by Dr. Anna Zakrisson.
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Retention is the process of holding water on the green roof, i.e., preventing water from draining off the green roof. Retention has been the focus of the hydrologic design of green roofs over the past few decades. Many people familiar with green roofs but less familiar with other types of stormwater management might not realize that most engineers consider retention the “free gift with purchase” rather than their central stormwater goal.
So, what is this odd thing, retention?
The secret of green roof water retention is green
Retention works like this: rain falls onto a relatively dry green roof, and the green roof absorbs the water, usually in pore spaces. Later, the sun comes out, and the plants use the rainwater to evapotranspire. This means that the water leaves the roof as vapor. Eventually, it rains again, and the process is repeated.
A surprising amount is going on there. Let’s unpack it!
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